Field of the Invention
The present invention relates to a device and a method for collimating or focusing a bunch of electrons, and a device and a method for emitting a bunch of relativistic electrons.
A “relativistic electron” should be understood to be an electron whose speed of displacement is not inconsiderable relative to the speed of light, notably whose speed is greater than 90% of the speed of light.
Description of Related Art
A so-called “laser-plasma” electron acceleration method is known. This method makes it possible to generate a bunch of electrons of high energy—conventionally a few hundreds of MeV—by focusing an intense laser pulse in a gas jet. The laser pulse creates a wave of electrical and magnetic fields which accelerate electrons present in the gas.
This method offers numerous advantages over the conventional electron acceleration techniques. In particular, this method may be implemented by means of a compact device, a distance of a few millimeters being sufficient to accelerate the electrons to an energy level of a few hundreds of MeV, whereas several tens of meters are needed to achieve such an energy level with conventional methods.
Moreover, the laser-plasma acceleration generates bunches of electrons that are extremely short, conventionally of the order of a few femtoseconds, and of very limited size, conventionally a few micrometers. Bunches of electrons with such characteristics are difficult to generate with conventional accelerators.
However, the bunches of electrons produced by laser-plasma acceleration exhibit a divergence which makes them difficult to use in practice.
This divergence of the bunches of electrons is difficult to correct with the known devices, such as the magnetic quadrupoles. In effect, the focusing force of a magnetic quadrupole is relatively weak. A quadrupole must therefore be placed several decimeters behind the source of the bunch of relativistic electrons, the bunch of electrons diverging accordingly between the source and the quadrupole, leading to a significant degradation of its emittance. The quadrupoles also have the disadvantage of being focusing only according to one of the two transverse directions—thus making it necessary to combine two or even three quadrupoles in order to obtain a suitable focusing.
Also known, notably from the article “A possible final focusing mechanism for linear colliders”, P. Chen, Particle Accelerators, 1987, Vol. 20, pp. 171-182, is a method for focusing a bunch of electrons using a plasma. According to this article, the bunch of electrons entering into a plasma generates therein, in its wake, a wave of focusing electrical fields. However, this method does not make it possible to focus all of the bunch of electrons, only a rear part of this bunch of electrons (in relation to the direction of propagation of the bunch of electrons). In the case of a very short bunch of electrons, as typically obtained by implementing a laser-plasma acceleration method, the part of the bunch of electrons located in the focusing zone is reduced to zero and the bunch of electrons is no longer focused at all by the wave of focusing electrical fields.
There is therefore a need for a focusing or collimation device that does not exhibit the abovementioned drawbacks and that notably makes it possible to focus or collimate a bunch of electrons obtained by laser-plasma acceleration.